ISO Week Number
Enter a date to get its ISO 8601 week number, ISO year, weekday name, and the Monday–Sunday date range for that week. Or enter a year and week number to see all seven dates. Runs entirely in your browser.
Find the ISO 8601 week number and ISO week-year for a calendar date, or expand a week number into its Monday-through-Sunday date range.
How to use the ISO week number tool
- Choose a date to see its ISO week, ISO year, weekday, and week start and end dates.
- Use Today to fill the current date quickly.
- Or enter an ISO year and week number from 1 to 53 to list all seven dates in that week.
- Copy any displayed week or date value when needed.
ISO week rules used here
ISO weeks start on Monday, and week 1 is the week containing January 4. Because of this rule, dates near New Year can belong to the previous or next ISO week-year instead of their calendar year.
Week-to-date conversion accepts week numbers 1 through 53. It calculates the Monday from January 4 and then lists seven UTC-based dates, so it does not validate whether a particular ISO year actually contains week 53.
Privacy and limitations
All week calculations run locally in your browser and this application does not send the selected dates to an external server. Results follow the implemented ISO 8601 week algorithm and are intended for ordinary calendar use rather than timezone-aware scheduling.
ISO Week Number FAQ
When does an ISO week start?
Monday. ISO weekdays run from Monday through Sunday.
Why can January 1 belong to the previous ISO year?
ISO week-years follow the week containing January 4, so dates around New Year can cross calendar-year boundaries.
What is ISO week 1?
The week containing January 4.
Can I enter week 53?
Yes. The input accepts values from 1 through 53, although not every ISO year actually has a valid week 53.
Are these calculations timezone-aware?
No. The week algorithm normalizes calendar dates and uses UTC arithmetic for week boundaries rather than a configurable timezone.